In vitro determination of ruminal protein degradability using [15N]ammonia to correct for microbial nitrogen uptake

An in vitro procedure was developed to estimate rate and extent of ruminal protein degradation using [15N]H3 to quantify uptake of protein degradation products for microbial protein synthesis. Incubations were conducted for 6 h in stirrer flasks with ruminal inoculum plus buffer, reducing solution, pectin, soluble carbohydrates, and added (15NH4)2SO4. Seven protein concentrates were tested in the system. Samples of media were analyzed for 15N enrichment of NH3, microbial N, and total solids N. Degradation rate was computed from net (i.e., protein-added minus blank) release of NH3 N plus net synthesis of microbial protein N; escape was estimated assuming ruminal passage rate = .06/h. Over the course of the incubations, pH was stable at 6.6, protozoal numbers increased slightly, and microbial protein content increased by more than 200%. Free AA had not accumulated at the end of the incubations. Microbial protein synthesis was a linear function (P < .001; r2 = .780) of extent of degradation. Mean degradation rates and ruminal escapes determined were, respectively, .569/h and 10% (casein), .148/h and 29% (solvent soybean meal), .036/h and 63% (expeller soybean meal), .026/h and 70% (low solubles fish meal), .063/h and 49% (high solubles fish meal), .034/h and 64% (corn gluten meal), and .050/h and 55% (roasted soybeans). Overall, degradation rates averaged 28% greater than those previously estimated using an inhibitor in vitro system; however, rates obtained for the fish meals using the 15N method were slower.